Endoprosthetic components, in particular for fusing vertebral bodies, are known. They are adapted in their geometry to the anatomy of the human vertebral body, are located between two vertebral bodies and completely or partially replace the intervertebral disk.
During a first phase of their duration in the human body, they typically keep the vertebral bodies at a distance and in an anatomically correct position solely by means of their mechanical properties. In a second phase, they promote the fusion and thus the growing together of the two surrounding vertebral bodies.
Known components for fusing vertebral bodies are based, for example, on metallic materials such as tantalum or titanium.
Disadvantages of these metallic materials are, for example:                Metallic abrasion and resulting negative effects on the human organism        Artifacts in imaging for medical diagnostics        Effects of aging and long-term performance, such as fatigue, corrosion and the release of metal ions, which can be toxic        
As a general problem has emerged more and more a risk of infection during surgery, which can be reduced with ceramic components.
Components based on plastics such as highly crosslinked PE (polyethylene) materials or PEEK (polyetheretherketone) are also known.
Disadvantages of plastic components are, for example:                Insufficient mechanical properties such as the breaking off of prongs or other constituents of the component, for example during installation.        Lack of presentability in current imaging processes (MRI, X-ray), thereby requiring the use of metallic markers.        Effects of aging and long-term performance, particularly material fatigue.        
Ceramic components based, for example, on silicon nitride, are also known. However, this class of materials was developed with an eye toward excellent high temperature properties—for instance for the machining of metal components for the automotive industry—and for the properties required for this use, such as strength, hardness and long-term stability, ranks rather in the midfield in comparison with other ceramic high-performance materials based on oxidic systems.
In addition, the material is relatively complicated, wherein needle-shaped silicon nitride is embedded in a glass matrix. The sintering of this material is accordingly laborious. The mechanical (post)processing, for example by means of grinding or polishing, is extremely demanding and difficult.
Moreover, components manufactured from silicon nitride have a rather dark coloration—gray to black—which for purely aesthetic reasons encounters a low level of acceptance in the medical field.
All of these disadvantages lead to increased costs in the manufacture of the components, which constitutes a further disadvantage.
A very important aspect of the use of ceramic components for the fusion of vertebral bodies is the generally high stiffness of these materials, which is substantially higher than that of human vertebral bodies.
In certain circumstances, this can cause so-called “stress shielding,” which can entail the breakdown of bone material and at least reduces or even eliminates the formation of new bone material. In this case, a fusion of vertebral bodies does not occur.
The principle of this effect can be explained in more detail as follows: Known ceramic cages are generally designed annularly and adapted to the form of the human vertebral body, whereby the ring consists of a monolithic, i.e. dense, solid and highly rigid ceramic. These cages often have a central cavity which is filled either with known bone replacement materials (autologous or allogenic), or have an artificial, porous, osseo-inductive or osseo-conductive structure which is generally considerably less stiff than the outer ring. In this area, the bone cells are to form new bone material, whereby the involved cells require an appropriate mechanical stimulus.
If, now, the forces caused by biomechanical stresses on the component are transmitted through the area with high rigidity, the mechanical stimulus in the central region of the component—that is, where the fusion is to take place—is absent, as the stress is shielded (“stress shielding”). Due to the lack of mechanical stimulus in this area, no bone formation and thus no fusion takes place.
This crucial disadvantage is to be solved by the present invention.
A further significant disadvantage of known solutions is the uncontrolled sinking-in of the components into the bone structure of the vertebral body. This uncontrolled sinking-in may occur if, due to the geometry of the component, a high point load is applied to a relatively soft bone substance.
The bone structure of a vertebral body is varied, for instance, the outer cortical bone substance is significantly denser and more solid than the inner cancellous bone substance. In addition, the bone structure of a human being is dependent on age, and of course also varies between individuals. Depending on the weight and activity level of the person, varying biomechanical stresses act on the vertebral column. As a result, uncontrolled sinking of the components into the vertebral body may occur, which can have diverse complications and consequences.
This disadvantage is to be solved by the present invention.